Whereas two babies switched at birth may not be the most original of inceptions, the rest of this 1990 classic is fairly unconventional!
One of the babies is "the Adversary, Destroyer of Kings, Angel of the Bottomless Pit, Great Beast that is called Dragon, Prince of this World, Father of Lies, Spawn of Satan and Lord of Darkness", which obviously entails quite a lot of sorting-out eleven years later, when Armageddon, the final battle between good and evil, is set to take place.
Terry Pratchett is a new acquaintance to me, though I had already - recently! - discovered that Neil Gaiman ('Coraline', 'American Gods') possesses an inventive energy quite akin to that displayed by Rowling in 'Harry Potter'. It was pretty irresistible for the first half of the book - especially the savage nitpicking on modern society; among the devil's inventions count for instance diet plans, oil spills, arms dealers and nouvelle cuisine - then the closer to Doomsday it got, the more I lost interest. Too many unimportant characters, perhaps?
All in all, though; a good read!